The goal is to elucidate the mechanisms underlying inactivation of peptide neurotransmitters in the CNS. Peptides are broadly distributed in the nervous system and appear to have important roles as chemical messengers. Although the roles of peptides in the CNS are beginning to be understood in detail, relatively little is known about peptide inactivation at the cellular level. I will focus my study on the bag cells and their target neurons in the abdominal ganglion of Aplysia californica. This system is well suited for the proposed studies because the biochemical, electrophysiological, and anatomical correlates of inactivation can be studied in the isolated ganglion. The bag cells are a homogeneous population of 800 neuroendocrine cells located in the abdominal ganglion; they release large amounts of peptides when they become electrically activated. Two bag cell peptides, egg laying hormone (ELH) and alpha-bag cell peptide (a-BCP) fulfill the major criteria for neurotransmitters. Each mediates a different subset of the effects that the bag cells have on target neurons in the abdominal ganglion. ELH is relatively resistant to inactivation; in contrast, a-BCP is quickly inactivated. The differences in the sensitivity of ELH and a-BCP to inactivation allow ELH to function both as a neurotransmitter within the abdominal ganglion, and as a blood-borne hormone, whereas a-BCP has only local effects. The inactivation of released a-BCP, measured by its recovery from the ganglion and its effects on target cells, can be blocked by arterial perfusion of peptidase inhibitors into the abdominal ganglion. These results suggest that a-BCP is inactivated by peptidases located in the interstitial and vascular spaces of the CNS. By using techniques such as 1) perfusing solutions of a-BCP through the ganglion, which mimics release of a-BCP and allows the study of inactivation in situ, 2) determining the changes in a-BCP structure by HPLC and amino acid sequence analysis, and 3) assaying the electrophysiological effects of perfusates on abdominal ganglion neurons, I will be able to more precisely define the mechanism(s) by which a-BCP is inactivated, and the consequences of this inactivation on electrical signaling in the ganglion. Insights gained by these experiments may help reveal some of the fundamental mechanisms underlying peptidergic neurotransmission. Furthermore, these studies may help develop ways to manipulate peptide levels in the central nervous system that are of clinical importance.